Device for determining and/or monitoring the level of a medium in a container

ABSTRACT

The invention relates to a vibration detector ( 1 ) for determining and/or monitoring the level of a medium in a container or for ascertaining the density of a medium in the container.  
     The object of the invention is to propose a vibration detector ( 1 ) that even under extreme process conditions and upon abrupt changes in process conditions still functions reliably.  
     This object is attained in that a tubular inner part ( 7 ) is provided, which is dimensioned such that it is positioned between the housing wall and the drive/receiving unit ( 6 ), and that the tubular inner part ( 7 ) is connected to the housing ( 2 ) or to the diaphragm ( 3 ).

[0001] The invention relates to a device for determining and/ormonitoring the level of a medium in a container or for ascertaining thedensity of a medium in the container, having a housing, a diaphragm, anoscillatable unit, a drive/receiving unit, and a regulating/evaluatingunit, wherein the diaphragm closes one face end of the housing, and theoscillatable unit is secured to the diaphragm, and the drive/receivingunit is disposed in the interior of the housing such that it causes thediaphragm and the oscillatable unit to oscillate, and detects theoscillations, and the regulating/evaluating unit, from the detectedoscillations, detects the attainment of the predetermined level orascertains the density of the medium.

[0002] Such vibration detectors have already become known in the mostvarious versions. The oscillatable unit is at least one oscillatory rod,which—as already noted—is secured directly to a diaphragm. The diaphragmis induced to oscillation via an electromechanical converter, such as apiezoelectric element. Because of the oscillations of the diaphragm, theoscillatable unit secured to the diaphragm also executes oscillations.

[0003] Vibration detectors embodied as fill level measuring instrumentsexploit the effect that the oscillation frequency and oscillationamplitude are dependent on the particular degree of coverage of theoscillatable unit: While the oscillatable unit in air can execute itsoscillations freely and without damping, it does undergo a change offrequency and amplitude as soon as it dips partway or all the way intothe product filling the container. From a predetermined frequency change(typically, the frequency rather than the amplitude is measured), anunambiguous conclusion can accordingly be drawn as to whether apredetermined fill level of a product in the container has beenattained. Fill level measuring instruments of this type are moreoverprimarily used as means to secure against overfilling or for the sake ofpreventing a pump from running empty.

[0004] Moreover, the damping of the oscillation of the oscillatingelement is affected by the density of the particular product as well.For a constant degree of coverage of the at least one oscillatingelement, there is a functional relationship with the density of theproduct, making vibration detectors extremely well suited for detectinga predetermined limit state as well as for detecting density.

[0005] In practice, to monitor and detect the level or density of theproduct in the container, the oscillations of the diaphragm are pickedup and converted into electrical reception signals. A piezoelectricelement is typically used for this. The electrical reception signals arethen evaluated by an electronic evaluator. In the case of leveldetermination, the electronic evaluator monitors the oscillationfrequency and/or the oscillation amplitude of the oscillatable unit andsignals that the state is either “sensor covered” or “sensor uncovered”,as soon as the measured values undershoot or overshoot a predeterminedreference value. This can be reported accordingly to the operating staffvisually and/or acoustically. Alternatively or in addition, a switchingoperation is tripped; an inflow or outflow valve, for instance, on thecontainer is then opened or closed.

[0006] From European Patent Disclosure EP 0 810 423 A2, a vibration-filllevel limit switch has been disclosed in which the piezoelectric elementis prestressed in the interior of the housing between a pressure plateand the center of the diaphragm. The pressure plate is braced on asecuring ring that is disposed in an annular groove on the inside of thehousing. A comparable embodiment of a vibration limit switch isdescribed in German Patent Disclosure DE 198 14 669 A1: Once again, thedrive element—a piezoelectric stack—is prestressed between the diaphragmand a yoke part. Here as well, the yoke part is braced directly on thehousing. For that purpose, stop parts are provided on the inner wall ofthe housing.

[0007] In the vibration detectors that have been disclosed, functionalproblems arise if the process conditions abruptly change, and especiallyif the process temperature suddenly rises. In the high-temperaturerange, process temperatures up to about 300° C. can furthermore occur.If the process temperature increases suddenly—that is, if a temperatureshock occurs—then the housing or so-called sensor tube expands muchfaster than the parts located inside the housing, which means thepiezoelectric element or piezoelectric stack and the pressure plate oryoke part. As a consequence, the extremely undesirable situation ensuesthat the piezoelectric drive is no longer in mechanical contact with thediaphragm, or that the set prestressing between the piezoelectric driveand the diaphragm is lost. The vibration detector then functions onlyincorrectly or not at all. Only once the parts located in the interiorof the housing have also been heated to the process temperature is thespecified prestressing between the piezoelectric drive and the diaphragmrestored, and the vibration detector is then fully functional again.Detector failures caused by a temperature shock can last on the order ofmagnitude of several minutes. This length of time is naturally entirelyunacceptable for a limit state detector that is used to prevent againstoverfilling or against running empty.

[0008] The object of the invention is to propose a device that functionsreliably even upon abrupt changes in the process temperature.

[0009] This object is attained in that a tubular inner part is provided,which is dimensioned such that it is positioned between the housing walland the drive/receiving unit, and that the tubular inner part isconnected to the housing or to the diaphragm.

[0010] Depending on the embodiment of the device of the invention, theparts disposed inside the housing, in particular the prestresseddrive/receiving unit, are completely or partly decoupled thermally fromthe housing by the tubular inner part. If a temperature shock occurs,the housing heats to the process temperature relatively fast. However,it takes a relatively long time until the tubular inner part and thedrive/receiving unit assume the altered process temperature. This is dueto the poor thermal conductivity of the air in the air gap between thetubular inner part and the housing wall and of the tubular inner partand the drive/receiving unit, which in the high-temperature range ispreferably made from piezoceramic materials. To guarantee high corrosionresistance, the housing and the tubular inner part of the vibrationdetector are moreover both made from stainless, acid-resistant steel.Because of the thermal decoupling of the housing and the drive/receivingunit provided according to the invention, the different thermalexpansion of the individual parts does not impair the proper function ofthe vibration detector.

[0011] In a preferred feature of the device of the invention, thetubular inner part is connected to the housing or to the diaphragm in adefined region. The fastening of the tubular inner part to the diaphragmor the housing—in the latter case, the connection is preferably made inthe vicinity of the diaphragm—is optimal, since in the event of a suddentemperature increase, the heat can spread evenly over the tubular innerpart and the drive/receiving unit.

[0012] In an advantageous refinement of the device of the invention, thetubular inner part, in the region of the connection of the tubular innerpart and the housing, has a beadlike ring, whose outer edge in theinstalled state is located essentially in the same plane as the outerwall of the housing. The connection between the tubular inner part andthe housing, or between the tubular inner part and the diaphragm, ispreferably done by welding. It is understood, however, that a screwconnection or any alternative way of connecting the individual parts toone another can also be employed.

[0013] In a preferred embodiment of the device of the invention, thedrive/receiving unit is a piezoelectric stack composed of at least twopiezoceramic disklike elements; one disklike element functions as adrive unit, while the other disklike element functions as a receivingunit. The most various drive/receiving units are already known from theprior art. It is self-evident that in conjunction with the presentinvention, all types of drive/receiving unit can be used. As especiallyadvantageous features, drive/receiving units that are used in vibrationdetectors offered for sale and distributed by the present Applicantunder the tradename “Liquiphant” can be named.

[0014] One favorable feature of the device of the invention is that thedrive/receiving unit is positioned between the diaphragm and a fasteningpart, the latter being braced on the inner wall of the housing or on thetubular inner part. The fastening part is preferably a pressure screw,with which the desired prestressing between the drive/receiving unit andthe diaphragm can be preset.

[0015] In the case where the fastening part is braced on the tubularinner part, care must be taken that the material comprising the tubularinner part have a coefficient of thermal expansion that is as similar aspossible to the coefficient of thermal expansion of the piezoelectricdrive and the fastening part. However, if a defined, sufficiently greatprestressing is selected, then it is entirely possible to selectmaterials for the individual parts that are not so well adapted to oneanother in terms of their coefficients of thermal expansion. Preferably,however, the housing, tubular inner part and fastening part, forinstance the pressure screw, are all of the same material, such asstainless and acid-resistant steel. In this connection, it can be notedthat the coefficient of thermal expansion of a piezoelectric materialand the coefficient of thermal expansion of stainless and acid-resistantsteel are approximately the same.

[0016] In the event that the fastening part is braced directly on thehousing, the vibration detector will function reliably well after atemperature shock only if the housing comprises a material that does notexpand, or expands only minimally, as a function of the temperature.Materials such as Inwar and Kowar have this property. The disadvantageof these materials is without doubt their limited applicability: Inwarand Kowar materials are known to be less corrosion-resistant.

[0017] If the fastening part is braced on the tubular inner part, thenan advantageous refinement proposes that the tubular inner part, on theend region remote from the diaphragm, has a female thread, and that thefastening part is a pressure screw that has a male thread correspondingto the female thread of the tubular inner part. It is understood thatfor locking the fastening part to the tubular inner part, the types offastening known from the prior art for prestressing the drive/receivingunit can also be used analogously.

[0018] A preferred feature of the device of the invention is that thedrive/receiving unit is part of a module that can be inserted into thehousing by the tubular inner part.

[0019] The invention will be explained in further detail in conjunctionwith the accompanying drawings. Shown are:

[0020]FIG. 1: a schematic illustration of a first embodiment of thedevice of the invention;

[0021]FIG. 2: an exploded view of the first embodiment of the device ofthe invention.

[0022]FIG. 3: an exploded view of the module that can be used in thedevice of the invention; and

[0023]FIG. 4: a schematic illustration of a second embodiment of thedevice of the invention.

[0024]FIG. 1 shows a schematic illustration of a first embodiment of thedevice according to the invention; in FIG. 2, this embodiment can beseen in an exploded view. The vibration detector 1 comprises a housing2, a tubular inner part 8, and an oscillatable unit 5, which is fastenedto a diaphragm 3. In the case shown, the oscillatable unit 5 comprisesan oscillating fork with two oscillatory rods 4. The tubular inner part7 has an annular bead 8, whose outer edge in the installed state isessentially flush with the outer surface of the housing 2. In the regionof the annular bead 8, the housing 2, tubular inner part 7 and diaphragm3 are welded to one another and to the oscillatable unit 5 secured tothe diaphragm. The advantage of joining the three parts together by aconnection such as a weld seam is considered to be that the tubularinner part, in particular, can expand freely and unhindered as afunction of temperature. The weld seam or connecting region ispreferably located in the vicinity of the diaphragm 3. This type ofconnection assures that the heat, in the event of an abrupt increase inthe process temperature, will spread uniformly over the tubular innerpart 7 and the piezoelectric drive, not shown separately in FIGS. 1 and2.

[0025] As can be seen from FIG. 2, a plurality of slits 23 are providedon the inner tube. This has advantages especially whenever the housing 2and diaphragm 3, with the oscillatable unit 5 fastened to the latter,are made from a different material from the tubular inner part 7. Inthat case, it can happen that the tubular inner part 7 will expand moreslowly than the other parts 2, 3, 5. The tubular inner part 7 then holdsthe other parts 2, 3, 5 quasi-firmly and seeks to prevent them fromexpanding. Strains that are beyond the allowable limit values can thenoccur in the material comprising the parts 2, 3, 5 or in the weld seam.The slits 23 lend the tubular inner part 7 a resilient effect; it canexpand outward more easily with the other parts 2, 3, 5. The strains inthe weld seam are reduced, so that they again come to be below theallowable limit value.

[0026] If as shown in these drawings the pressure screw 15 thatprestresses the piezoelectric drive is firmly fastened to the tubularinner part 7, the parts disposed in the interior of the housing 2 expandevenly and independently of the housing 2. The set prestressing of thepiezoelectric stack therefore stays at least within the scope ofspecified tolerances. As a result, it is assured that the vibrationdetector 1 will always furnish reliably correct measured values evenunder extreme conditions. As already noted earlier herein, it isespecially advantageous if the materials that comprise the tubular innerpart 7 and the fastening part, in this case the pressure screw 15, havea coefficient of thermal expansion similar to that of the piezoelectricmaterial comprising the drive/receiving unit.

[0027]FIG. 3 shows an exploded via of the module 16 that is preferablyused in conjunction with the present invention. The piezoelectric stack10 is disposed in the tubular inner part 7. This piezoelectric stack 10represents the preferred embodiment of the drive/receiving unit 6 forthe vibration detector 1. The individual piezoelectric disklike elements20 of the piezoelectric stack 10 preferably comprise a piezoceramicmaterial, which is also suitable for use in the high-temperature range.

[0028] The piezoelectric stack 10 is in contact with the diaphragm 3 viathe die 18. The die 18 facilitates centering the piezoelectric stack 10relative to the diaphragm 3. The contact lugs 19, which are formed onthe disklike piezoelectric elements 20 and extended to the outside,serve to provide electrical contact.

[0029] The pressure screw 15 has a male thread 14, which corresponds tothe female thread 13 of the tubular inner part 7. The piezoelectricstack 10 is positioned via the pressure screw 15 in such a way that itacts on the diaphragm 3 with a defined prestressing.

[0030] The two connection parts 21 likewise serve to provide electricalcontact. Via the shaped part 22, the electrical connecting lines, notshown separately in FIG. 3, are additionally stabilized. Moreover, theshaped part 22 facilitates the installation—in the case shown, tightlyscrewing—the module 16 in the housing 2.

[0031] In FIG. 4, a schematic illustration of a second embodiment of thevibration detector 1 of the invention can be seen. While in theembodiment shown in FIGS. 1, 2 and 3, the tubular inner part 7 isfastened to the housing 2, in this second embodiment the tubular innerpart 7 is fastened directly to the diaphragm 5. List of ReferenceNumerals 1 Vibration detector 2 Housing 3 Diaphragm 4 Oscillatory rod 5Oscillatable unit 6 Drive/receiving unit 7 Tubular inner part 8 Beadlikering 9 Disklike element 10 Piezoelectric stack 11 Fastening part 12Inner wall of the housing 13 Female thread 14 Male thread 15 Pressurescrew 16 Module 17 Air gap 18 Die 19 Contact lug 20 Piezoceramicdisklike element 21 Connection part 22 Shaped part 23 Slit

1. A device for determining and/or monitoring the level of a medium in acontainer or for ascertaining the density of a medium in the container,having a housing, a diaphragm, an oscillatable unit, a drive/receivingunit, and a regulating/evaluating unit, wherein the diaphragm closes oneface end of the housing, and the oscillatable unit is secured to thediaphragm, and the drive/receiving unit is disposed in the interior ofthe housing such that it causes the diaphragm and the oscillatable unitto oscillate, and detects the oscillations, and theregulating/evaluating unit, from the detected oscillations, detects theattainment of the predetermined level or ascertains the density of themedium, characterized in that a tubular inner part (7) is provided,which is dimensioned such that it is positioned between the housing walland the drive/receiving unit (6), and that the tubular inner part (7) isconnected to the housing (2) or to the diaphragm (3).
 2. The device ofclaim 1, characterized in that the tubular inner part (7) is welded orscrewed to the housing (2) or to the diaphragm (3) in a defined region.3. The device of claim 1 or 2, characterized in that the tubular innerpart (7), in the region of the connection of the tubular inner part (7)and the housing (2) or diaphragm (3), has a beadlike ring (8), whoseouter edge in the installed state is located essentially in the sameplane as the outer wall of the housing (2).
 4. The device of claim 1, 2or 3, characterized in that the connection between the tubular innerpart (7) and the housing (2) or diaphragm (3) is located in the endregion, toward the diaphragm (3), of the housing (2).
 5. The device ofclaim 1, characterized in that the drive/receiving unit (6) is apiezoelectric stack (10) composed of at least two piezoceramic disklikeelements (9).
 6. The device of claim 1, characterized in that thedrive/receiving unit (6) or the piezoelectric stack (10) is positionedbetween the diaphragm (3) and a fastening part (11), which part isbraced on the housing (2) or on the tubular inner part (7).
 7. Thedevice of claim 6, characterized in that the tubular inner part (7), onthe end region remote from the diaphragm (3), has a female thread (13),and that the fastening part (11) is a pressure screw (15) that has amale thread (14) corresponding to the female thread (13) of the tubularinner part (7).
 8. The device of claim 1, 5 or 6, characterized in thatthe drive/receiving unit (6) or the piezoelectric stack (10) is part ofa module (16) that is insertable into the housing (2) by the tubularinner part (7).